1. Field of Invention
The present invention relates to a variable frequency drive system. More particularly, the present invention relates to a three-phase medium voltage variable frequency driving system.
2. Description of Related Art
In the controlling of a motor machine or an induction motor, it is an important issue to adjust the motor speed. The traditional direct current (DC) speed regulating technology adopted in a conventional motor machine has limited applications due to big volume and high failure rate of hardware.
A variable-frequency drive (VFD) is an electric drive element using the variable-frequency technology and the electronic technology to control an alternating current (AC) motor by changing the frequency and amplitude of a motor operation power supply.
The VFD is used for changing the AC power supply frequency and the amplitude of the induction motor, so as to change its motional magnetic field period, thereby achieving the aim of controlling the rotation rate of the induction motor smoothly. The emergence of VFD simplifies the complex speed-regulating control. The combination of the VFD and the AC induction motor replaces most tasks that originally can only be completed by using a DC motor, so that the volume of a circuit system is decreased and the maintenance ratio is reduced.
Currently, the medium-voltage variable frequency speed-regulating system is applied widely and has broad prospects in aspects such as a large-scale wind generator, a pump, drafting and gearing. The medium-voltage variable frequency speed-regulating system needs to have the following main functions: safety, fast speed, frequency control at a wide range; a good power factor at the electrical grid side; good input and output current harmonic waves and the like.
Meanwhile, due to the high requirements on the withstand voltage of a switch element in a medium-voltage (referring to a voltage between 1 kV-35 kV, such as 6 kV in a common application) system, the current most-common medium-voltage variable frequency speed-regulating system mostly use a multilevel cascade scheme. Referring to FIGS. 1 and 2, FIG. 1 illustrates a schematic view of a medium-voltage variable frequency speed-regulating system 300 adopting a multilevel architecture in the prior art, and FIG. 2 illustrates a schematic interior circuit diagram of the power unit 320 of the conventional medium-voltage variable frequency speed-regulating system 300 in FIG. 1. As shown in FIG. 1, in the medium-voltage variable frequency speed-regulating system 300, a multistage transformer can transform the high voltage at the electrical grid side into a plurality of secondary low voltages. Each secondary winding is connected with a separate power unit 320. As shown in FIG. 2, each power unit 320 completes the change from rectification to inversion, so as to implement a variable frequency speed-regulating function. The transformer is incorporated not only to solve issues regarding the withstand voltage of power devices, but also to solve issues about current harmonic waves at the electrical grid side. Taking the multistage transformer as an example, the multistage transformer can transform a high input voltage of the three-phase electrical grid (at the primary side) into a low operation voltage at the secondary side. Each winding at the secondary side is respectively coupled with a separate power unit. Each power unit completes the change from rectification to inversion for a low operation voltage, so as to implement a variable frequency speed-regulating function. Through the arrangement of the above mentioned multistage transformer, the issue that the power unit cannot withstand high voltage is solved, and the issue about current harmonic waves at the primary side is also solved. However, the multistage transformer arranged in the above mentioned conventional medium-voltage variable frequency speed-regulating system is of large volume and high weight, which leads to high cost and complex design. Thus, it is an important researching issue regarding how to use other speed-regulating system structures to omit the arrangement of the transformer while achieving the same performance.
FIG. 3 illustrates a structure of a conventional medium-voltage variable frequency speed-regulating system 400. The structure illustrated in FIG. 3 is a structure of the so-called back-to-back variable frequency speed-regulating system in the industry, which is an important structure of the medium-voltage speed-regulating system without a transformer. This back-to-back variable frequency speed-regulating structure 400 of the medium-voltage variable frequency speed-regulating system is based on a diode-clamping three-level topology. The multilevel design decreases the withstand voltage of a switching tube by a half. The structure of the rectification side is symmetrical with that of the inversion side, which can effectively control the power factor and harmonic waves at the electrical grid side and adjust the four-quadrant operation of the motor, so as to realize energy feedback. This energy feedback characteristic has significant meaning for applications of loads such as a lifter.
However, the above mentioned conventional medium-voltage variable frequency speed-regulating system without a transformer is still provided with a large number of switch elements, and the system has a relatively complex structure and still high cost.